The Naval Research
Laboratory's (NRL's) Large Angle
Spectrometric Coronagraph (LASCO),
which is flying aboard the
Solar and Heliospheric Observatory
(SOHO), has been credited
with discovering its 500th comet. This
achievement makes LASCO
the most productive comet finder ever.
Most of the comets that
LASCO has discovered are in the family
of Kreutz sun-grazers,
which are thought to derive from a single
massive comet that
had fragmented into many pieces, because they
all have very
similar orbits. The original comet was probably
last seen about
12,000 years ago.

LASCO is a wide-field
white light and spectrometric
coronagraph developed for flight
aboard ESA-NASA's SOHO
mission. The launch took place on December
2, 1995. The mission
began routine observations in May 1996 upon
completion of
commissioning activities after arriving in an orbit
about the
L1 Lagrangian point. In this orbit one million miles
from Earth
toward the sun, the spacecraft is always in sunlight
and able
to continuously view the sun. LASCO detected its 500th
comet on
August 12, 2002. Before the development of space-borne
coronagraphs in the 1960s, only about ten sun-grazers had ever
been detected. The NRL white light coronagraph, SOLWIND aboard
the USAF Space Test Program P78-1 satellite, recorded images
of
another member of the Kreutz group in August, 1979. This was
the first comet ever discovered using satellite instrumentation.
SOLWIND and the coronagraph on the Solar Maximum Mission discovered
a total of 16 comets from 1979-1989. The increased sensitivity
of LASCO detects about one comet a week, compared to the earlier
rate of about one comet every 8 months, explains Dr. Russell
Howard, head of NRL's Solar Physics Branch.

LASCO views the space
around the sun, looking for outbursts of solar activity by blocking
out the solar disk to create an artificial eclipse. This artificial
eclipse allows the Sun's very faint outer atmosphere ­ the
corona ­ to be studied. LASCO is able to detect comets that
would normally be lost in the glare of the sun. Most of these
sun-grazers are eventually vaporized in the solar atmosphere.
Other non-sungrazing comets have been observed such as in the
spring of 1996, when LASCO recorded unprecedented images of the
bright Comet Hyakutake making its close approach to the
sun.

LASCO was not built
for the purpose of detecting
comets. Its main job is to detect
and even anticipate the
origin of coronal mass ejections (CMEs),
the solar sources of
space weather. CMEs, which were first discovered
byNRL in 1971,
can reach velocities ten times higher than solar
wind, and when
directed toward Earth, can have a serious impact
on the earth's
magnetic field and upper atmosphere. CMEs can
cause aurorae and
geomagnetic storms, disrupt communications
and military
systems, and even damage electrical power grids.

LASCO is a set of
three coronal telescopes, using nested, concentric fields of
view at the center of which a dark occulting disk blocks out
direct light from the sun's brilliant photosphere; thus simulating
a continuous total eclipse of the sun. The instrument observes
the inner, the intermediate, and the outermost regions of the
sun's corona over an unprecedented range of distances. LASCO
was the first instrument to record coronal features from just
above the solar limb, where the coronal glow is about one million
times fainter than the solar disk, all the way out to nearly
30
solar radii (nearly 13 million miles) above the surface, where
the ever-fainter corona blends into and becomes the solar
wind.

High-resolution charged-coupled-device
cameras in
each telescope have provided detailed images in near-real
time
to the world-wide-web, with exceptional dynamic range, while
large digital memories and high-speed microprocessors have supported
extensive onboard image processing and image data compression
that allow transmission of up to 10 full coronal images per
hour.

Amateur astronomers
in Europe have discovered the
great majority of the LASCO comets.
They download the LASCO
real time images that are put onto the
web and analyze them for
transient features. Many false detections
have occurred because
the CCD detectors are also sensitive to
energetic particles
such as cosmic rays. The tracks caused by
energetic particles
look like comet tracks, but occur randomly
from image to image.
Thus they don't appear to be in an orbit
toward the
sun.

The U.S. Naval Research Laboratory provides the advanced scientific capabilities required to bolster our country's position of global naval leadership. The Laboratory, with a total complement of approximately 2,500 personnel, is located in southwest Washington, D.C., with other major sites at the Stennis Space Center, Miss., and Monterey, Calif. NRL has served the Navy and the nation for over 90 years and continues to advance research further than you can imagine. For more information, visit the NRL website or join the conversation on Twitter, Facebook, and YouTube.

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